2 A tour of git: the basics
6 This document is a modified version of a document originally titled
7 "Distributed revision control with Mercurial" and originally authored
8 by Bryan O’Sullivan. The original document was obtained from
9 <http://hgbook.red-bean.com/>.
11 Copyright © 2006, 2007 Bryan O’Sullivan.
13 This material may be distributed only subject to the terms and
14 conditions set forth in version 1.0 of the Open Publication
15 License. Please refer to Appendix D for the license text.
17 As this is a modified version, the name of Bryan O'Sullivan is used
18 only to properly credit him with the original text. The appearance of
19 his name here explicitly does not assert or imply his endorsement of
20 this modified document.
22 Portions Copyright © 2007 Carl Worth.
24 Changes made by Carl include the following:
27 * Convert from HTML to markdown source syntax
28 * Eliminate all content except Chapter 2 and Appendix D
29 * Eliminate line numbers from examples
30 * Modified to describe git instead of mercurial
32 The source of this modified version can be obtained via git:
34 git clone git://cworth.org/git/hgbook-git
38 git clone http://cworth.org/git/hgbook-git
40 and can be [browsed online](http://git.cworth.org/git/hgbook-git)
42 ### 2.1 Installing git on your system
44 Prebuilt binary packages of git are available for many popular
45 operating systems. These make it easy to start using git on your
50 Because each Linux distribution has its own packaging tools, policies,
51 and rate of development, it’s difficult to give a comprehensive set of
52 instructions on how to install git binaries. The version of
53 git that you will end up with can vary depending on how active
54 the person is who maintains the package for your distribution.
56 To keep things simple, I will focus on installing git from the
57 command line under the most popular Linux distributions. Most of these
58 distributions provide graphical package managers that will let you
59 install git with a single click. The package name to look for is
60 often git, but is sometimes git-core, (due to an unfortunate name
61 with git, meaning GNU Interactive Tools).
65 apt-get install git-core
81 apt-get install git-core
85 A git-core package is available through
86 [macports](http://macports.org). Once macports is enabled, the command
93 Git has long been available as part of cygwin, and works reasonably
94 well in that environment. Some people find cygwin a particularly
95 inelegant approach to running git and would prefer a "native"
96 solution. To this end, the [msysgit
97 project](http://code.google.com/p/msysgit/) is rapidly putting
98 together a solution including various packages with full
99 installers. These include GitMe, a package to install the entire
100 development environment necessary to work on improving the msysgit
101 port of git, and WinGit, a package for installing just git itself
102 without the development environment, (still in Alpha as of September
105 ### 2.2 Getting started
107 To begin, we’ll use the “git version” command to find out whether git
108 is actually installed properly. Versions 1.5 and newer of git are much
109 more friendly to new users than versions 1.4 and older. If you aren't
110 yet running version 1.5 or newer, it's highly recommended that you
116 #### 2.2.1 Built-in help
118 Git provides a built-in help system. This is invaluable for those
119 times when you find yourself stuck trying to remember how to run a
120 command. If you are completely stuck, simply run “git help”; it will
121 print a brief list of commonly-used commands, along with a description
122 of what each does. If you ask for help on a specific command (such as
123 "git help init"), it prints more detailed information. [XXX: Does "git
124 help <foo>" work universally as a built-in or does it expect man to be
125 present and just call out to "man git-<foo>"?]
127 [XXX: The original hgbook includes the complete output of "hg
128 help init" at this point. I'm not including the corresponding
129 "git help init" output as it would be excessively long. The
130 description alone is quite reasonable, (other than a
131 not-too-helpful aside about the obsolete git-init-db command),
132 but it only comes after a full screen's worth of options
133 details. Might it make sense to have a more summarized help
134 output for "git help <foo>" than all of the documentation
135 available for git-<foo>? And perhaps alos provide a "git -v
136 help" similar to "hg -v help" for more?]
138 ### 2.3 Working with a repository
140 In git, everything happens inside a repository. The repository
141 for a project contains all of the files that “belong to” that project,
142 along with a historical record of the project’s files.
144 There’s nothing particularly magical about a repository; it is simply
145 a directory tree in your filesystem that git treats as
146 special. You can rename or delete a repository any time you like,
147 using either the command line or your file browser.
149 #### 2.3.1 Creating a local copy of a remote repository
151 As suggested, a repository can be copied through normal file-copying
152 commands. But git also provides a "git clone" tool for copying a
153 repository. This provides a means of copying a repository over the
154 network, and is also useful with a local repository since it is much
155 more efficient than creating a normal copy, (creating a local clones
158 We've assembled a simple repository that will be used in the examples
159 throughout this chapter. Go ahead and clone this repository now so
160 that you will be able to follow along:
162 $ git clone git://cworth.org/git/hello
163 Initialized empty Git repository in /tmp/hello/.git/
164 remote: Generating pack...
165 remote: Done counting 15 objects.
166 remote: Deltifying 15 objects...
167 remote: 100% (15/15) done
168 remote: Total 15 (delta 2), reused 15 (delta remote: 2)
169 Indexing 15 objects...
171 Resolving 2 deltas...
174 If for some reason you are prevented from talking on the git: port,
175 then there is also the capability to clone a repository (less
176 efficiently) over http:
178 $ git clone http://cworth.org/git/hello
179 Initialized empty Git repository in /tmp/hello/.git/
180 Getting alternates list for http://cworth.org/git/hello
181 Getting pack list for http://cworth.org/git/hello
182 Getting index for pack 04ecb061314ecbd60fa0610ecf55a1cbf85ea294
183 Getting pack 04ecb061314ecbd60fa0610ecf55a1cbf85ea294
184 which contains a1a0e8b392b17caf50325498df54802fe3c03710
185 walk a1a0e8b392b17caf50325498df54802fe3c03710
186 walk 72d4f10e4a27dbb09ace1503c20dbac1912ee451
187 walk 13ed136b983a9c439eddeea8a1c2076cffbb685f
188 walk 0a633bf58b45fcf1a8299d3c82cd1fd26d3f48f2
189 walk db7117a9dd9a6e57e8632ea5848e1101eee0fbde
191 If our clone succeeded, we should now have a local directory called
192 hello. This directory will contain some files.
196 drwxr-xr-x 3 cworth cworth 4096 2007-09-27 16:40 hello
200 These files have the same contents and history in our repository as
201 they do in the repository we cloned.
203 Every git repository is complete, self-contained, and
204 independent. It contains its own private copy of a project’s files and
205 history. A cloned repository remembers the location of the repository
206 it was cloned from, but it does not communicate with that repository,
207 or any other, unless you tell it to.
209 What this means for now is that we’re free to experiment with our
210 repository, safe in the knowledge that it’s a private “sandbox” that
211 won’t affect anyone else.
213 #### 2.3.2 What’s in a repository?
215 When we take a more detailed look inside a repository, we can see that
216 it contains a directory named .git. This is where git keeps all
217 of its metadata for the repository.
221 . .. .git hello.c Makefile
223 The contents of the .git directory and its subdirectories are private
224 to git. Every other file and directory in the repository is
225 yours to do with as you please.
227 To introduce a little terminology, the .git directory is the “real”
228 repository, and all of the files and directories that coexist with it
229 are said to live in the working directory. An easy way to remember the
230 distinction is that the repository contains the history of your
231 project, while the working directory contains a snapshot of your
232 project at a particular point in history.
234 ### 2.4 A tour through history
236 One of the first things we might want to do with a new, unfamiliar
237 repository is understand its history. The “git log” command gives us a
241 commit a1a0e8b392b17caf50325498df54802fe3c03710
242 Author: Bryan O'Sullivan <bos@serpentine.com>
243 Date: Tue Sep 6 15:43:07 2005 -0700
247 commit 72d4f10e4a27dbb09ace1503c20dbac1912ee451
248 Author: Bryan O'Sullivan <bos@serpentine.com>
249 Date: Tue Sep 6 13:15:58 2005 -0700
251 Get make to generate the final binary from a .o file.
253 commit 13ed136b983a9c439eddeea8a1c2076cffbb685f
254 Author: Bryan O'Sullivan <bos@serpentine.com>
255 Date: Tue Sep 6 13:15:43 2005 -0700
257 Introduce a typo into hello.c.
259 commit 0a633bf58b45fcf1a8299d3c82cd1fd26d3f48f2
260 Author: Bryan O'Sullivan <mpm@selenic.com>
261 Date: Fri Aug 26 01:21:28 2005 -0700
265 commit db7117a9dd9a6e57e8632ea5848e1101eee0fbde
266 Author: Bryan O'Sullivan <mpm@selenic.com>
267 Date: Fri Aug 26 01:20:50 2005 -0700
269 Create a standard "hello, world" program
271 By default, this command prints a brief paragraph of output for each
272 change to the project that was recorded. In git terminology, we
273 call each of these recorded events a commit.
275 The fields in a record of output from “git log” are as follows.
277 * commit This field consists of a string of 40 hexadecimal characters.
278 This is a unique identifier for referring to particular commits.
279 * Author The identity of the person who authored the commit. This
280 field consist of two sub-fields for the user's name and email
281 address, (or at least an email-like idenitifer). Note that git
282 stores a separate "Committer" field for the person who commited
283 the change, (since often an author will email a change to a
284 maintainer that commits it). The "git log" command doesn't display
285 the Committer, but other git tools do.
286 * Date The date and time on which the commit was authored, (again
287 stored separately from the date the change was committed).
288 timezone in which it was created. (The date and time are displayed
289 in the timezone of the person who created the commit.)
290 * commit message The text message that the creator of the commit
291 entered to describe the commit, (generally a one-line summary
292 followed by more supporting text).
294 The default output printed by “git log” is purely a summary; it is
295 missing a lot of detail.
297 #### 2.4.1 Commits, revisions, and talking to other people
299 As English is a notoriously sloppy language, and computer science has
300 a hallowed history of terminological confusion (why use one term when
301 four will do?), revision control has a variety of words and phrases
302 that mean the same thing. If you are talking about git history
303 with other people, you will find that what we have called a “commit”
304 is often called a "revision". In other systems, a similar notion
305 is referred to as a "changeset". You might even see abbreviations of
306 these terms such as "rev", "change", or even "cset".
308 While it may not matter much what word you use to refer to the concept
309 of “a commit”, it's important to know how to name “a specific
310 commit”. We have already seen one means of referring to a particular
311 commit, the 40-character hexadecimal string shown by "git log". These
312 commit identifiers are powerful because they are permanent, unique
313 identifiers that always identify the same commit in any copy of a
314 repository. If two users are examining a working directory associated
315 with the same commit identifier, then those two users have precisely
316 the same contents in all files, and exactly the same history leading
319 So there are places where it is often important to archive the
320 complete commit identifier, (perhaps in bug-tracking systems to
321 indicate a specific commit that fixes a bug, for example). But often,
322 in more casual settings, it's more convenient to use abbreviated
323 commit identifiers. Git accept any unique prefix of a commit
324 identifier, (and for reasonably-sized project the first 8 or 10
325 characters are almost always unique).
327 And unlike the permanent commit identifiers, git also provides
328 transient means of identifying commits. In fact, in day-to-day use of
329 git, you will probably use these names more than commit
330 identifiers. One example is branch names, (such as the default
331 "master" branch in any git repository), or any project-specific branch
332 names such as "stable", "experimental", or "crazy-insane-changes". Git
333 also provides a special name "HEAD" which always refers to the current
336 #### 2.4.2 Naming related commits
338 Git offers simple ways to name revisions that are related to
339 particular revisions in the history. One syntax is the ~ suffix which
340 refers to the parent of a commit, or if followed by a number, to the
341 Nth parent. For example, since "HEAD" refers to the most recent commit
342 in the current branch, "HEAD~", refers to the previous commit, and
343 "HEAD~2" refers to two commits back in the history.
345 Another useful syntax is .. which can be used to specify a range of
346 commits. So "origin..master" specifies everything that has been
347 committed to master since it diverged from origin.
349 #### 2.4.3 Viewing specific revisions
351 You can use "git log" to explore the range syntax just introduced. For
352 example, to see a list of the most recent 3 revisions you can use
353 "HEAD~3..", (the destination of the range is implicitly HEAD in this
357 commit a1a0e8b392b17caf50325498df54802fe3c03710
358 Author: Bryan O'Sullivan <bos@serpentine.com>
359 Date: Tue Sep 6 15:43:07 2005 -0700
363 commit 72d4f10e4a27dbb09ace1503c20dbac1912ee451
364 Author: Bryan O'Sullivan <bos@serpentine.com>
365 Date: Tue Sep 6 13:15:58 2005 -0700
367 Get make to generate the final binary from a .o file.
369 commit 13ed136b983a9c439eddeea8a1c2076cffbb685f
370 Author: Bryan O'Sullivan <bos@serpentine.com>
371 Date: Tue Sep 6 13:15:43 2005 -0700
373 Introduce a typo into hello.c.
375 #### 2.4.4 Other log filters
377 Besides filtering by commit identifiers, git allows you to easily
378 filter the log output according to which files (or directories) are
379 modified by listing them after "--" wihch is necessary to distinguish
380 commit names from file names:
382 $ git log -- Makefile
383 commit 72d4f10e4a27dbb09ace1503c20dbac1912ee451
384 Author: Bryan O'Sullivan <bos@serpentine.com>
385 Date: Tue Sep 6 13:15:58 2005 -0700
387 Get make to generate the final binary from a .o file.
389 commit 0a633bf58b45fcf1a8299d3c82cd1fd26d3f48f2
390 Author: Bryan O'Sullivan <mpm@selenic.com>
391 Date: Fri Aug 26 01:21:28 2005 -0700
395 And "git log" can also filter based on the dates at which commits were
398 $ git log --since="2 weeks ago" --until="yesterday"
400 Another useful option is -n or --max-count which, unsurprisingly,
401 limits the maximum number of commits to be displayed.
403 #### 2.4.5 More detailed information
405 While the default information printed by “git log” is useful if you
406 already know what you’re looking for, you may need to see more details
407 of the change, such as the "diffstat" information with --stat:
409 $ git log --stat --max-count=3
410 commit a1a0e8b392b17caf50325498df54802fe3c03710
411 Author: Bryan O'Sullivan <bos@serpentine.com>
412 Date: Tue Sep 6 15:43:07 2005 -0700
417 1 files changed, 2 insertions(+), 6 deletions(-)
419 commit 72d4f10e4a27dbb09ace1503c20dbac1912ee451
420 Author: Bryan O'Sullivan <bos@serpentine.com>
421 Date: Tue Sep 6 13:15:58 2005 -0700
423 Get make to generate the final binary from a .o file.
426 1 files changed, 2 insertions(+), 0 deletions(-)
428 commit 13ed136b983a9c439eddeea8a1c2076cffbb685f
429 Author: Bryan O'Sullivan <bos@serpentine.com>
430 Date: Tue Sep 6 13:15:43 2005 -0700
432 Introduce a typo into hello.c.
435 1 files changed, 1 insertions(+), 1 deletions(-)
437 Or perhaps you'd like to see the actual patch content of each change,
438 which you can get with -p. That commit with the word typo in its name
439 looks suspicous, so let's tak a closer look. Remember that we can name
440 it as master~3, HEAD~3, or any prefix of its commit identifier, (such
443 $ git log -p -n 1 13ed136b
444 commit 13ed136b983a9c439eddeea8a1c2076cffbb685f
445 Author: Bryan O'Sullivan <bos@serpentine.com>
446 Date: Tue Sep 6 13:15:43 2005 -0700
448 Introduce a typo into hello.c.
450 diff --git a/hello.c b/hello.c
451 index ed55ec0..80b260c 100644
456 int main(int argc, char **argv)
458 - printf("hello, world!\n");
459 + printf("hello, world!\");
463 Of course, wanting to see all this information for a single commit is
464 such a common operation that it's given its own name in git, "git
465 show". So "git show 13ed136b" is a much easier way to get exactly the
469 commit 13ed136b983a9c439eddeea8a1c2076cffbb685f
470 Author: Bryan O'Sullivan <bos@serpentine.com>
471 Date: Tue Sep 6 13:15:43 2005 -0700
473 Introduce a typo into hello.c.
475 diff --git a/hello.c b/hello.c
476 index ed55ec0..80b260c 100644
481 int main(int argc, char **argv)
483 - printf("hello, world!\n");
484 + printf("hello, world!\");
488 ### 2.5 All about command options
490 Let’s take a brief break from exploring git commands to discuss
491 a pattern in the way that they work; you may find this useful to keep
492 in mind as we continue our tour.
494 Git has a consistent and straightforward approach to dealing
495 with the options that you can pass to commands. It follows the
496 conventions for options that are common to modern Linux and Unix
499 * Most options have long names. For example, as we’ve already seen,
500 the “git log" command accepts a --max-count=<number> option.
501 * Some options have short, single-character names. Often these are
502 aliases for long commands, (such as "-n <number>" instead of
503 --max-count=<number>), but sometimes the option exists in
504 short-form with no long-form equivalent, (such as -p). [XXX: It
505 wouldn't hurt to fix this by adding --patch, etc. right?]
506 * Long options start with two dashes (e.g. --max-count), while short
507 options start with one (e.g. -n).
509 * Option naming and usage is consistent across commands. For
510 example, every command that lets you specify a commit identifier
511 or range will accept the same expressions, (HEAD~3,
512 origin..master, 72d4f10e, etc), while any command that can be
513 limited by paths will accept the same expressions ("-- doc/
516 Many commands that print output of some kind can be made more quiet by
517 passing the -q or --quiet options.
519 ### 2.6 Making and reviewing changes
521 Now that we have a grasp of viewing history in git, let’s take a
522 look at making some changes and examining them.
524 The first thing we’ll do is isolate our experiment in a repository of
525 its own. We use the “git clone” command, but we don’t need to clone a
526 copy of the remote repository. Since we already have a copy of it
527 locally, we can just clone that instead. This is much faster than
528 cloning over the network, and cloning a local repository uses less
529 disk space in most cases, too.
532 $ git clone hello my-hello
533 Initialized empty Git repository in /tmp/my-hello/.git/
536 [XXX We say "empty" here, (presumably from the git-init part),
537 but shouldn't the command also report the succesful clone
538 which makes it non-empty? And what the heck does "0 blocks"
541 As an aside, it’s often good practice to keep a “pristine” copy of a
542 remote repository around, which you can then make temporary clones of
543 to create sandboxes for each task you want to work on. This lets you
544 work on multiple tasks in parallel, each isolated from the others
545 until it’s complete and you’re ready to integrate it back. Because
546 local clones are so cheap, there’s almost no overhead to cloning and
547 destroying repositories whenever you want.
549 Alternatively, you can achieve much the same effect by creating
550 multiple branches in a single repository, (but we won't go into detail
551 on how to do that in this chapter). Some people greatly appreciate
552 having multiple branches in a single repository rather than having
553 many repositories cluttering up their filesystem. Other people prefer
554 the ability to have working-tree changes, and intermediate build
555 files, etc. each isolated in a separate repository per branch. Both
556 modes are very well-supported by git, so it's really a matter of which
557 you find most appropriate at any time given your tastes and project
560 In our my-hello repository, we have a file hello.c that contains the
561 classic “hello, world” program. Let’s use the ancient and venerable
562 sed command to edit this file so that it prints a second line of
563 output. (I’m only using sed to do this because it’s easy to write a
564 scripted example this way. Since you’re not under the same constraint,
565 you probably won’t want to use sed; simply use your preferred text
566 editor to do the same thing.)
568 $ sed -i '/printf/a\\tprintf("hello again!\\n");' hello.c
570 The “git status” command will tell us what git knows about the files
577 # Changed but not updated:
578 # (use "git add <file>..." to update what will be committed)
582 no changes added to commit (use "git add" and/or "git commit -a")
584 We see that “git status” command prints a line with "modified" for
585 hello.c. The “git status” command will not print any output for files
586 that have not been modified.
588 Notice that we didn’t need to inform git that we were going to modify
589 the file before we started, or that we had modified the file after we
590 were done; it was able to figure this out itself.
592 It’s a little bit helpful to know that we’ve modified hello.c, but we
593 might prefer to know exactly what changes we’ve made to it. To do
594 this, we use the “git diff” command.
597 diff --git a/hello.c b/hello.c
598 index 9a3ff79..6d28887 100644
602 int main(int argc, char **argv)
604 printf("hello, world!\");
605 + printf("hello again!\n");
609 ### 2.7 Recording changes in a new commit
611 We can modify files, build and test our changes, and use “git status”
612 and “git diff” to review our changes, until we’re satisfied with what
613 we’ve done and arrive at a natural stopping point where we want to
614 record our work in a new commit.
616 The “git commit” command lets us create a new changeset; we’ll usually
617 refer to this as “making a commit” or “committing”.
619 #### 2.7.1 Setting up a username
621 When you try to run “git commit” for the first time, it might not do
622 exactly what you want. Git records your name and address with each
623 change that you commit, (as both author and committer unless you tell
624 it otherwise), so that you and others will later be able to tell who
625 made each change. Git tries to automatically figure out a sensible
626 name and address to attribute to both author and committer. It will
627 attempt each of the following methods, in order, (stopping for each field as soon as a value is found):
629 1. If you specify a --author option to the “git commit” command on
630 the command line, followed by a "Real Name <email@example.com>"
631 string, then this name and addresss will be used for the author
632 fields. The committer fields will still be determined as
633 below. This option is very helpful for when applying a commit
634 originally authored by someone other than yourself.
635 2. If any of the GIT_AUTHOR_NAME, GIT_AUTHOR_EMAIL,
636 GIT_COMMITTER_NAME, or GIT_COMMITER_EMAIL environment variables
637 are set, then those values will be used for the corresponding
639 3. If you have a file in your home directory called .gitconfig, with
640 name or email settings in the [user] section, then these values
641 will be used to set any remaining author and committer
642 fields. For more details on the contents of this file, refer to
644 4. If you have a file in the local repository called .git/config,
645 again with name or email settings in the [user] section, then
646 these values will be used to set any remaining author and
648 5. If you have set the EMAIL environment variable, this will be used
649 to set author and committer email addresses if still unset.
650 6. git will query your system to find out your real name from
651 available GECOS field and your username, hostname, and domain to
652 construct an email address, (or at least an identifier resembling
655 If all of these mechanisms fail, "git commit" will fail, printing an
656 error message instructing you how to use "git config" to tell git your
657 name and email address.
659 You should think of the GIT_AUTHOR/COMMITER_NAME/EMAIL environment
660 variables and the --author option to the “git commit” command as ways
661 to override git’s default selection. For normal use, the simplest and
662 most robust way to set your information is by creating a .gitconfig
663 file, (either manually or with the "git config" command); see below
666 ##### Creating a git configuration file
668 To set your name and email address, just use the following commands:
670 git config --global user.name "Your Name"
671 git config --global user.email "you@example.com"
673 The --global option means that this command will set global
674 information, (affecting all repositories on this machine), in the
675 .gitconfig file in your home directory. Alternately, you could omit
676 the --global which would make the change take effect only in the local
677 repository. This is convenient if you want to have different email
678 addresses associated with different projects, for example.
680 Of course, git's configuration file is a simple-to-edit plain-text
681 file, so instead of using the above commands, you can also just edit
682 the files directly. Use your favorite editor to create a file called
683 .gitconfig in your home directory, (or if you ran the above commands
684 then it will be there already). The initial contents of your
685 .gitconfig should look like this.
687 # This is a git configuration file.
690 email = you@example.com
692 Similarly, you can make a repository-specific configuration by editing
693 .git/config in the local repository. It will already have some
694 sections present, (created by the "git clone"), just add a [user]
697 The “[user]” line begins a section of the config file, so you can read
698 the “name = ...” line as meaning “set the value of the name item in
699 the user section”. This is the same notion expressed with the
700 "user.name" syntax on the git-config command line. A section
701 continues until a new section begins, or the end of the file. Git
702 ignores empty lines and treats any text from “#” to the end of a line
705 ##### Choosing a user name
707 You can use any text you like as the value of the name and email
708 configuration items, since this information is for reading by other
709 people, not for interpreting by git. It is conventional to use a valid
710 email address, but some, (notably Linus Torvalds, the original author
711 of git), actually like the default user@hostname convention that git
712 falls back on without any additional information. There's no
713 requirement that the email address actually be valid, and perhaps it's
714 useful to be reminded which machine was used to create particular
717 #### 2.7.2 Writing a commit message
719 When we commit a change, git drops us into a text editor to
720 enter a message that will describe the modifications we’ve made in
721 this commit. This is called the commit message. It will be a record
722 for readers of what we did and why, and it will be printed by “git log”
723 after we’ve finished committing.
727 Note: The -a on the command-line instructs git to commit all changes
728 to tracked files. Without this, "git commit" will only commit changes
729 that have been previously staged for committing with "git add
730 file". The most common usage is to commit with "git commit -a" and
731 only use "git add file; git commit" when there is a need to commit
732 only some subset of changes that have been made.
734 The editor that the “git commit” command drops us into will contain an
735 empty line, followed by a number of lines starting with “#”.
738 # Please enter the commit message for your changes.
739 # (Comment lines starting with '#' will not be included)
741 # Changes to be committed:
742 # (use "git reset HEAD <file>..." to unstage)
747 git ignores the lines that start with “#”; it uses them only
748 to tell us which files it’s recording changes to. Modifying or
749 deleting these lines has no effect.
751 #### 2.7.3 Writing a good commit message
753 A good commit message will generally have a single line that
754 summarizes the commit, a blank line, and then one or more pargraphs
755 with supporting detail. Since many tools only print the first line of
756 a commit message by default, it’s important that the first line stands
759 One example of a first-line-only viewer is "git log
760 --pretty=short". Other examples include graphical history viewers such
761 as gitk and gitview, and web-based viewers such as gitweb and cgit.
763 Here’s a real example of a commit message that doesn’t follow
764 this guideline, and hence has a summary that is not readable.
766 $ git log --pretty=short
767 commit 3ef5535144da88a854f7930503845cd44506c2e2
768 Author: Censored Person <censored.person@example.org>
770 include buildmeister/commondefs. Add an exports and install
772 As far as the remainder of the contents of the commit message are
773 concerned, there are no hard-and-fast rules. git itself doesn’t
774 interpret or care about the contents of the commit message, though
775 your project may have policies that dictate a certain kind of
778 My personal preference is for short, but informative, commit messages
779 that tell me something that I can’t figure out with a quick glance at
780 the output of “git log -p".
782 #### 2.7.4 Aborting a commit
784 If you decide that you don’t want to commit while in the middle of
785 editing a commit message, simply exit from your editor without saving
786 the file that it’s editing. This will cause nothing to happen to
787 either the repository or the working directory.
789 #### 2.7.5 Admiring our new handiwork
791 Once we’ve finished the commit, we can use the “git show” command to
792 display the commit we just created. As discussed previously, this
793 command produces output that is identical to “git log -p”, but for
794 only a single revision, (and the most recent revision by default):
797 commit 018cfb742be6176443ffddac454e593e802ddf3e
798 Author: Carl Worth <cworth@cworth.org>
799 Date: Thu Sep 27 23:55:00 2007 -0700
801 Added an extra line of output.
803 If I would have been clever I would have fixed that old typo
806 diff --git a/hello.c b/hello.c
807 index 9a3ff79..6d28887 100644
811 int main(int argc, char **argv)
813 printf("hello, world!\");
814 + printf("hello again!\n");
818 Note that you will not see the same commit identifier for your commit,
819 even if the change you made is identical to mine. The commit
820 identifier incorporates not only the contents of the files, but commit
821 message, the author and committer names and emails, and the author and
822 commit dates. (OK, so now you probably know enough to be able to guess
823 the right command to produce a commit with exactly the commit
824 identifier shown above. Can you do it?)
826 #### 2.7.6 Fixing up a broken commit (before anyone else sees it)
828 So now that we've cloned a local repository, made a change to the
829 code, setup our name and email address, and made a commit with a
830 careful message, we're just about ready to share our change with the
831 world. But wait, we forgot to try to compile it didn't we?
834 cc -c -o hello.o hello.c
835 hello.c:10:9: warning: missing terminating " character
836 hello.c:10:9: warning: missing terminating " character
837 hello.c: In function ‘main’:
838 hello.c:10: error: missing terminating " character
839 hello.c:11: error: expected ‘)’ before ‘;’ token
840 hello.c:13: warning: passing argument 1 of ‘printf’ makes pointer from integer without a cast
841 hello.c:13: error: expected ‘;’ before ‘}’ token
842 make: *** [hello.o] Error 1
844 Oh look. The code's broken and doesn't compile. We don't want to share
845 code in this state. For situations where you notice one tiny detail
846 that got left out of the last commit, (a silly syntax error, a
847 misspelling in a comment or commit messsage), git provides a very
848 handy tool for just changing the last commit.
850 So fix that typo, (a missing 'n' between the '\' and the '"'), with
851 your editor or with something like this:
853 sed -i 's/\\"/\\n"/' hello.c
855 And then you can just amend the previous commit rather than creating a
856 new one with the --amend option to "git commit":
858 $ git commit -a --amend
860 Note that we use -a to include the code change here. And that helps
861 point out a situation where "git commit" is useful without the -a
862 option, "git commit --amend" is a useful command for amend just the
863 last commit message, without committing any new code changes, even if
864 some files have been modified in the working tree.
866 And here's the final result:
869 commit 839b58d021c618bd0e1d336d4d5878a0082672e6
870 Author: Carl Worth <cworth@cworth.org>
871 Date: Thu Sep 27 23:55:00 2007 -0700
873 Added an extra line of output and fixed the typo bug.
875 diff --git a/hello.c b/hello.c
876 index 9a3ff79..ca750e0 100644
881 int main(int argc, char **argv)
883 - printf("hello, world!\");
884 + printf("hello, world!\n");
885 + printf("hello again!\n");
889 I can't help but point out that this really was a poor example for
890 --amend. The end result is a single commit that does two independent
891 things, (fixes one bug and adds one new feature). It's much better to
892 create a code history where each commit makes an independent change,
893 (and as small as possible). This is important for several reasons:
895 * Small changes are easier to review
897 * Independent changes are easier to split up if only part of the
898 series gets accepted "upstream" for one reason or another.
900 * The smaller the changes are the more useful the history will be
901 when actually using the history, not just viewing it. This is
902 particularly important when doing "git bisect"---that's a powerful
903 tool for isolating the single commit that introduces a bug. And
904 it's much more powerful if the commit it isolates is as small as
907 So it's a good thing this document is available under a license that
908 allows for distribution of modified versions. Someone should clean up
909 the --amend example to not teach bad habits like I did above. [Note:
910 All this bad-habit stuff was introduced by me, and was not present in
911 Bryan's original chapter. -Carl]
913 ### 2.8 Sharing changes
915 We mentioned earlier that repositories in git are
916 self-contained. This means that the commit we just created exists
917 only in our my-hello repository. Let’s look at a few ways that we can
918 propagate this change into other repositories.
920 #### 2.8.1 Pulling changes from another repository
922 To get started, let’s clone our original hello repository, which does
923 not contain the change we just committed. We’ll call our temporary
924 repository hello-pull.
927 $ git clone hello hello-pull
928 Initialized empty Git repository in /tmp/hello-pull/.git/
931 We could use the “git pull” command to apply changes from my-hello to
932 our master branch in hello-pull. However, blindly pulling unknown
933 changes into a repository is a somewhat scary prospect. The "git pull"
934 command is coneptually the combination of two commands, "git fetch"
935 and "git merge"; we can run those separately to examine the changes
936 before applying them locally. First we do the fetch:
939 $ git fetch ../my-hello
940 remote: Generating pack...
941 Unpacking 3 objects...
943 remote: Done counting 5 objects.
944 Result has 3 objects.
945 Deltifying 3 objects...
946 100% remote: (3/3) done
947 Total 3 (delta 1), reused 0 (delta 0)
949 The fetched commits (or commit in this case) are available as the name
950 FETCH_HEAD. [XXX: Shouldn't git-fetch print that name out to the user
951 if the user didn't provide a specific branch name to fetch into.] And
952 the difference between what we had before and what exists on
953 FETCH_HEAD can easily be examined with the ..FETCH_HEAD range
956 $ git log ..FETCH_HEAD
957 commit 839b58d021c618bd0e1d336d4d5878a0082672e6
958 Author: Carl Worth <cworth@cworth.org>
959 Date: Thu Sep 27 23:55:00 2007 -0700
961 Added an extra line of output and fixed the typo bug.
963 Since these commits actually exist in the local repository now, we
964 don't need to fetch or pull them from the remote repository again---we
965 can now use "git merge" to apply the previously fetched commits. (A
966 mercurial user might notice here that git does not have the race
967 condition between "hg incoming" and "hg pull" that mercurial has since
968 the commits are fetched only once.)
970 $ git merge FETCH_HEAD
971 Updating a1a0e8b..839b58d
974 1 files changed, 2 insertions(+), 1 deletions(-)
976 Notice that "git merge" reports that our branch pointer has been
977 updated from a1a0e8b to 839b58d. Also, this is a "fast forward"
978 meaning that the new commits are a linear sequence on top of the
979 commit we already hand. In other words, there wasn't any divergence
980 between these two repositories so no actual "merge" commit was
983 This separation of fetch and merge is useful when you need to
984 carefully review some changes before applying them. But often you're
985 in a situation where you know you trust the remote repository and you
986 simply want to pull those changes as conveniently as possible, (no
987 extra commands, no typing a magic name like FETCH_HEAD). This is the
988 case when the tracking upstream development of a project with git. And
989 in that case, the above steps are as simple as just executing "git
990 pull". So let's repeat all that the simpler way:
993 $ git clone hello hello-tracking
994 Initialized empty Git repository in /tmp/hello-tracking/.git/
997 $ git pull ../my-hello
998 remote: Generating pack...
999 remote: Done counting 5 objects.
1000 Result has 3 objects.
1001 Deltifying 3 objects...
1002 Unpacking 3 objects...
1003 remote: 100% (3/3) done
1004 Total 3 (delta 1), reused 0 (delta 0)
1006 Updating a1a0e8b..839b58d
1009 1 files changed, 2 insertions(+), 1 deletions(-)
1011 It should be plain to see that the "git pull" command really did the
1012 combined sequence of "git fetch" and "git merge". Also, if you want to
1013 pull from the same repository you cloned from originally, (which is
1014 the common case for the upstream-tracking scenario), then "git pull"
1015 with no explicit repository is suffcient, and it will default to
1016 pulling from the same repository as the original clone.
1018 [XXX: The structure of the preceding section follows that of the
1019 original hgbook. But an alternate structure that arranged to pull from
1020 the originally cloned repository (as would be common) would allow for
1021 more straightforward use of git's features. For example, instead of
1022 the silly FETCH_HEAD stuff it would allow for "git fetch" and "git log
1023 master..origin" to be a very nice replacement for "hg
1024 incoming". Similarly, below, "git log origin..master" would make a
1025 nice replacement for "hg outgoing" which is something I didn't offer
1026 at all. One could also use git's remotes with the myriad repositories
1027 as used here, but it would require doing things like "git remote add
1028 <some-name> ../hello-pull" and that seems like a bit much to introduce
1029 for a turorial of this level. If nothing else, if the above section
1030 seems a little intimidating, understand that it's because things are
1031 not presented in the most natural "git way", (and I'm a little too
1032 tired to fix it tonight).]
1034 #### 2.8.2 Checking out previous revisions
1036 If any users of mercurial are reading this, they might wonder if
1037 there's a need for the equivalent of "hg update" after doing a "git
1038 pull". And the answer is no. Unlike mercurial, "git pull" and "git
1039 merge" will automatically update the workind-directory files as
1042 But there's another function provided by "hg update" which is to
1043 update the working-directory files to a particular revision. In git,
1044 this functionality is provided by the "git checkout" command. To
1045 checkout a particular branch, tag, or an arbitrary revions, simply
1046 give the appropriate name to the "git checkout" command. For example,
1047 to examine the files as they existed before the original typo
1048 introduction, we could do:
1050 $ git checkout 0a633bf5
1051 Note: moving to "0a633bf5" which isn't a local branch
1052 If you want to create a new branch from this checkout, you may do so
1053 (now or later) by using -b with the checkout command again. Example:
1054 git checkout -b <new_branch_name>
1055 HEAD is now at 0a633bf... Create a makefile
1057 The note that git gives us is to indicate that we are checking out a
1058 non-branch revision. This is perfectly fine if we are just exploring
1059 history, but if we actually wanted to use this revision as the basis
1060 for new commits, we would first have to create a new branch name as it
1063 For now, let's return back to the tip of the master branch by just
1064 checking it out again:
1066 $ git checkout master
1067 Previous HEAD position was 0a633bf... Create a makefile
1068 Switched to branch "master"
1070 #### 2.8.3 Pushing changes to another repository
1072 Git lets us push changes to another repository, from the repository
1073 we’re currently visiting. As with previous examples, above, we’ll
1074 first create a temporary repository to push our changes into. But
1075 instead of using "git clone", this time we'll use "git init" to make a
1076 repository from an empty directory. We do this to create a "bare"
1077 repository which is simply a repository that has no working-directory
1078 files associated with it. In general, you should only push to bare
1085 Initialized empty Git repository in /tmp/hello-push/
1087 And then we'll go back to our my-hello repository to perform the
1088 push. Since this is our very first push into this repository we need
1089 to tell git which branches to push. The easiest way to do this is to
1090 use --all to indicate all branches:
1093 $ git push ../hello-push --all
1094 updating 'refs/heads/master'
1095 from 0000000000000000000000000000000000000000
1096 to 839b58d021c618bd0e1d336d4d5878a0082672e6
1098 Done counting 18 objects.
1099 Deltifying 18 objects...
1101 Writing 18 objects...
1103 Total 18 (delta 3), reused 0 (delta 0)
1104 Unpacking 18 objects...
1106 refs/heads/master: 0000000000000000000000000000000000000000 -> 839b58d021c618bd0e1d336d4d5878a0082672e6
1108 For subsequent pushes we don't need to specify --all as "git push"
1109 will push all branches that exist in both the local and remote
1112 What happens if we try to pull or push changes and the receiving
1113 repository already has those changes? Nothing too exciting.
1115 $ git push ../hello-push
1116 Everything up-to-date
1118 #### 2.8.4 Sharing changes over a network
1120 The commands we have covered in the previous few sections are not
1121 limited to working with local repositories. Each works in exactly the
1122 same fashion over a network connection; simply pass in a URL or an ssh
1123 host:/path/name specification instead of a local path.
1126 Open Publication License
1128 Version 1.0, 8 June 1999
1130 ### D.1 Requirements on both unmodified and modified versions
1132 The Open Publication works may be reproduced and distributed in whole
1133 or in part, in any medium physical or electronic, provided that the
1134 terms of this license are adhered to, and that this license or an
1135 incorporation of it by reference (with any options elected by the
1136 author(s) and/or publisher) is displayed in the reproduction.
1138 Proper form for an incorporation by reference is as follows:
1140 Copyright (c) year by author’s name or designee. This material may be
1141 distributed only subject to the terms and conditions set forth in the
1142 Open Publication License, vx.y or later (the latest version is
1143 presently available at
1144 [http://www.opencontent.org/openpub/][http://www.opencontent.org/openpub/]).
1146 The reference must be immediately followed with any options elected by
1147 the author(s) and/or publisher of the document (see section D.6).
1149 Commercial redistribution of Open Publication-licensed material is
1152 Any publication in standard (paper) book form shall require the
1153 citation of the original publisher and author. The publisher and
1154 author’s names shall appear on all outer surfaces of the book. On all
1155 outer surfaces of the book the original publisher’s name shall be as
1156 large as the title of the work and cited as possessive with respect to
1161 The copyright to each Open Publication is owned by its author(s) or
1164 ### D.3 Scope of license
1166 The following license terms apply to all Open Publication works,
1167 unless otherwise explicitly stated in the document.
1169 Mere aggregation of Open Publication works or a portion of an Open
1170 Publication work with other works or programs on the same media shall
1171 not cause this license to apply to those other works. The aggregate
1172 work shall contain a notice specifying the inclusion of the Open
1173 Publication material and appropriate copyright notice.
1175 Severability. If any part of this license is found to be unenforceable
1176 in any jurisdiction, the remaining portions of the license remain in
1179 No warranty. Open Publication works are licensed and provided “as is”
1180 without warranty of any kind, express or implied, including, but not
1181 limited to, the implied warranties of merchantability and fitness for
1182 a particular purpose or a warranty of non-infringement.
1184 ### D.4 Requirements on modified works
1186 All modified versions of documents covered by this license, including
1187 translations, anthologies, compilations and partial documents, must
1188 meet the following requirements:
1190 1. The modified version must be labeled as such.
1191 2. The person making the modifications must be identified and the
1192 modifications dated.
1193 3. Acknowledgement of the original author and publisher if
1194 applicable must be retained according to normal academic citation
1196 4. The location of the original unmodified document must be identified.
1197 5. The original author’s (or authors’) name(s) may not be used to
1198 assert or imply endorsement of the resulting document without the
1199 original author’s (or authors’) permission.
1201 ### D.5 Good-practice recommendations
1203 In addition to the requirements of this license, it is requested from
1204 and strongly recommended of redistributors that:
1206 1. If you are distributing Open Publication works on hardcopy or
1207 CD-ROM, you provide email notification to the authors of your
1208 intent to redistribute at least thirty days before your
1209 manuscript or media freeze, to give the authors time to provide
1210 updated documents. This notification should describe
1211 modifications, if any, made to the document.
1212 2. All substantive modifications (including deletions) be either
1213 clearly marked up in the document or else described in an
1214 attachment to the document.
1215 3. Finally, while it is not mandatory under this license, it is
1216 considered good form to offer a free copy of any hardcopy and
1217 CD-ROM expression of an Open Publication-licensed work to its
1220 ### D.6 License options
1222 The author(s) and/or publisher of an Open Publication-licensed
1223 document may elect certain options by appending language to the
1224 reference to or copy of the license. These options are considered part
1225 of the license instance and must be included with the license (or its
1226 incorporation by reference) in derived works.
1228 1. To prohibit distribution of substantively modified versions
1229 without the explicit permission of the author(s). “Substantive
1230 modification” is defined as a change to the semantic content of
1231 the document, and excludes mere changes in format or
1232 typographical corrections.
1234 To accomplish this, add the phrase “Distribution of substantively
1235 modified versions of this document is prohibited without the
1236 explicit permission of the copyright holder.” to the license
1239 2. To prohibit any publication of this work or derivative works in
1240 whole or in part in standard (paper) book form for commercial
1241 purposes is prohibited unless prior permission is obtained from
1242 the copyright holder.
1244 To accomplish this, add the phrase “Distribution of the work or
1245 derivative of the work in any standard (paper) book form is
1246 prohibited unless prior permission is obtained from the copyright
1247 holder.” to the license reference or copy.